This invention relates to the production of cellulose from lignocellulosic biomass, and in particular to process whereby cellulose is separated from other constituents of lignocellulosic biomass so as to make the cellulose available as a chemical feedstock and/or accessible to enzymatic hydrolysis for conversion to sugar.
The possibility of producing sugar and other products from cellulose has received much attention. This attention is due to the availability of large amounts of cellulosic feedstock, the need to minimize burning or landfilling of waste cellulosic materials, and the usefulness of sugar and cellulose as raw materials substituting for oil-based products.
Natural cellulosic feedstocks typically are referred to as xe2x80x9cbiomassxe2x80x9d. Many types of biomass, including wood, paper, agricultural residues, herbaceous crops, and municipal and industrial solid wastes, have been considered as feedstocks. These biomass materials primarily consist of cellulose, hemicellulose, and lignin bound together in a complex gel structure along with small quantities of extractives, pectins, proteins, and ash. Due to the complex chemical structure of the biomass material, microorganisms and enzymes cannot effectively attack the cellulose without prior treatment because the cellulose is highly inaccessible to enzymes or bacteria. This inaccessibility is illustrated by the inability of cattle to digest wood with its high lignin content even though they can digest cellulose from such material as grass. Successful commercial use of biomass as a chemical feedstock depends on the separation of cellulose from other constituents.
The separation of cellulose from other biomass constituents remains problematic, in part because the chemical structure of lignocellulosic biomass is not yet well understood. See, e.g., ACS Symposium Series 397, xe2x80x9cLignin, Properties and Materialsxe2x80x9d, edited by W. G. Glasser and S Sarkanen, published by the American Chemical Society, 1989, which includes the statement that xe2x80x9c[l]ignin in the true middle lamella of wood is a random three-dimensional network polymer comprised of phenylpropane monomers linked together in different ways. Lignin in the secondary wall is a nonrandom two-dimensional network polymer. The chemical structure of the monomers and linkages which constitute these networks differ in different morphological regions (middle lamella vs. secondary wall), different types of cell (vessels vs. fibers), and different types of wood (softwoods vs. hardwoods). When wood is delignified, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived.xe2x80x9d
The separation of cellulose from other biomass constituents is further complicated by the fact that lignin is intertwined and linked in various ways with cellulose and hemicellulose. In this complex system, it is not surprising that the xe2x80x9cseverity indexxe2x80x9d commonly used in data correlation and briefly described below, can be misleading. This index has a theoretical basis for chemical reactions (such as hydrolysis) involving covalent linkages. In lignocellulose, however, there are believed to be four different mechanisms of non-covalent molecular association contributing to the structure: hydrogen bonding, stereoregular association, lyophobic bonding, and charge transfer bonding. Bonding occurs both within and between components. As temperature is increased, bonds of different types and at different locations in the polymeric structure will progressively xe2x80x9cmeltxe2x80x9d, thereby disrupting the structure and mobilizing the monomers and macro-molecules.
Many of these reactions are reversible, and on cooling, re-polymerization can occur with deposits in different forms and in different locations from their origins. This deposition is a common feature of various conventional high temperature cellulosic biomass separation techniques. Furthermore, at higher temperatures in acid environments, mobilization of lignin is in competition with polymer degradation through hydrolysis and decomposition impacting all lignocellulosic components. As a result, much effort has been expended to devise xe2x80x9coptimumxe2x80x9d conditions of time and temperature that maximize the yield of particular desired products. These efforts have met with only limited success.
Known techniques for the conversion of biomass directly to sugar or other chemicals include concentrated acid hydrolysis, weak acid hydrolysis and pyrolysis processes. These processes are not known to have been demonstrated as feasible at commercial scale under current economic conditions or produce cellulose as either a final or intermediate product.
Conventional processes for separation of cellulose from other biomass components include processes used in papermaking such as the alkaline kraft process most commonly used in the United States and the sulphite pulping process most commonly used in central Europe. There are additional processes to remove the last traces of lignin from the cellulose pulp. This is referred to as xe2x80x9cbleachingxe2x80x9d and a common treatment uses a mixture of hot lye and hydrogen peroxide. These technologies are well established and economic for paper making purposes, but have come under criticism recently because of environmental concerns over noxious and toxic wastes. These technologies are also believed to be too expensive for use in production of cellulose for use as chemical raw material for low value products.
The use of organic solvents in cellulose production has recently been commercialized. These processes also are expensive and intended for production of paper pulp.
Many treatments have been investigated which involve preparating crude cellulose at elevated temperature for enzymatic hydrolysis to sugar. Investigators have distinguished particular process variations by such names as xe2x80x9csteam explosionxe2x80x9d, xe2x80x9csteam cookingxe2x80x9d, xe2x80x9cpressure cooking in waterxe2x80x9d, xe2x80x9cweak acid hydrolysisxe2x80x9d, xe2x80x9cliquid hot water pretreatmentxe2x80x9d, and xe2x80x9chydrothermal treatmentxe2x80x9d. The common feature of these processes is wet cooking at elevated temperature and pressure in order to render the cellulosic component of the biomass more accessible to enzymatic attack. In recent research, the importance of lignin and hemicellulose to accessibility has been recognized.
Steam cooking procedures typically involve the use of pressure of saturated steam in a reactor vessel in a well-defined relationship with temperature. Because an inverse relationship generally exists between cooking time and temperature, when a pressure range is stated in conjunction with a range of cooking times, the shorter times are associated with the higher pressures (and temperatures), and the longer times with the lower pressures. As an aid in interpreting and presenting data from steam cooking, a xe2x80x9cseverity indexxe2x80x9d has been widely adopted and is defined as the product of treatment time and an exponential function of temperature that doubles for every 10xc2x0 C. rise in temperature. This function has a value of 1 at 100xc2x0 C.
It is known that steam cooking changes the properties of lignocellulosic materials. Work on steam cooking of hardwoods by Mason is described in U.S. Pat. Nos. 1,824,221; 2,645,633; 2,294,545; 2,379,899; 2,379,890; and 2,759,856. These patents disclose an initial slow cooking at low temperatures to glassify the lignin, followed by a very rapid pressure rise and quick release. Pressurized material is blown from a reactor through a die (hence xe2x80x9csteam explosionxe2x80x9d), causing defibration of the wood. This results in the xe2x80x9cfluffyxe2x80x9d, fibrous material commonly used in the manufacture of Masonite(trademark) boards and Cellotex(trademark) insulation.
More recent research in steam cooking under various conditions has centered on breaking down the fiber structure so as to increase the cellulose accessibility. One such pretreatment involves an acidified xe2x80x9csteam explosionxe2x80x9d followed by chemical washing. This treatment may be characterized as a variant of the weak acid hydrolysis process in which partial hydrolysis occurs during pretreatment and the hydrolysis is completed enzymatically downstream. One criticism of this technique is that the separation of cellulose from lignin is incomplete. This makes the process only partially effective in improving the accessibility of the cellulose to enzymatic attack. Incomplete separation of cellulose from lignin is believed to characterize all steam cooking processes disclosed in prior art.
Advanced work with steam cooking in the United States has been carried out at the National Renewable Energy Laboratory in Golden, Colo. U.S. Pat. Nos. 5,125,977; 5,424,417; 5,503,996; 5,705,369; and 6,022,419 to Torget, et al. incorporated herein by reference, involve the minimization of acid required in the production of sugar from cellulose by acid hydrolysis in processes that may also include the use of cellulase enzymes. These patents teach the use of an acid wash of solids in the reaction chamber at the elevated temperature and pressure conditions where hemicellulose and lignin are better decomposed and mobilized. The use of acid is tied to the goal of sugar production by hydrolysis. The focus of Torget""s work appears to be acid treatments and hydrolysis and does not claim to produce high purity cellulose that is a principal objective of the present invention.
A common feature of acid hydrolysis, acid pretreatment, and chemical paper pulping is the generation of large quantities of waste chemicals that require environmentally acceptable disposal. One proposed means of waste disposal is as a marketable byproduct. Thus wallboard has been suggested as a potential use for the large quantities of gypsum produced in acid hydrolysis and acid pretreatment. This potential market is believed illusory since the market for cheap sugar is so vast that any significant byproduct will quickly saturate its more limited market.
There remains a pressing need for a process to provide low cost cellulose for subsequent conversion to glucose sugar by enzymatic hydrolysis. However, the presence of lignin in cellulosic biomass increases dramatically the amount of enzyme needed, thereby imposing unacceptably high conversion costs. Economics demand a process by which substantially pure cellulose can be produced for only a few cents per pound. Mainstream scientific and engineering efforts to utilize lignocellulosic biomass have been unable to achieve this goal over several decades. The challenge is to find a process that solves or avoids the problems of cost, chemical wastes, the clean separation of lignocellulosic components, and the unwanted degradation of said components.
Ignored by the mainstream effort is a process referred to as xe2x80x9cwet oxidationxe2x80x9d. This is a mature technology used for the disposal of liquid and toxic organic wastes. The process involves exposing a slurry of organic material to oxygen at elevated temperature and pressure even higher than that used in steam cooking. The result is destruction of the organic material and its conversion to carbon dioxide and water. While the effectiveness of wet oxidation in the chemical modification of organic matter has been demonstrated at commercial scale, the severity of chemical breakdown in waste disposal applications leaves few useful products.
The use of wet oxidation in the pretreatment of lignocellulosic biomass is known. In one described process, wet oxidation occurs at relatively low temperatures (40xc2x0 C.) and extends over 2 days. In other uses of wet oxidation in the pretreatment of lignocellulosic biomass, there is no control of pH, so acids formed in the process essentially create a variant of the mild acid pretreatment process.
In other wet oxidation work with wheat straw to recover hydrolyzed hemicellulose, process temperatures were maintained from 150xc2x0 to 200xc2x0 C. and pH was maintained at above 5 with sodium carbonate. Lower pHs were avoided to minimize decomposition and the formation of chemicals toxic in downstream processes. The separation of cellulose from lignin was not a stated goal in this work, and it is believed that the chemical conditions were not appropriate for such a separation. Perhaps the greatest deficiency of this work is that the entire biomass was subjected to the same treatment for the entire processing time. Thus a compromise was needed with consideration given to both the most reactive and the least reactive components. The resulting xe2x80x9coptimizedxe2x80x9d procedure fails to satisfy the requirements for commercialization because of component degradation and low yields.
Thus it can be seen that neither technologies for paper making, for acidified steam cooking, nor for wet oxidation as presently practiced can fill the need for commercially economical techniques for preparation of high purity cellulose from cellulosic biomass which do not produce objectionable waste streams.
Accordingly, one object of the present invention is to provide a lower cost and environmentally benign process for the separation of cellulose from other constituents of cellulosic biomass.
Another object of this invention is to produce at high yield and in a chemically active state cellulose that is substantially free of lignin, hemicellulose, and extractives that are other constituents of biomass.
According to the present invention, it has been found that relatively pure cellulose can be produced if lignocellulosic materials first are treated with steam to partially hydrolyze the hemicellulose to soluble oligomers and then are washed with alkaline hot water containing dissolved oxygen to remove these hydrolysis products and to decompose, mobilize and remove lignin, extractives, and residual hemicellulose.
A preferred method of the present invention involves the production of purified cellulose containing less than 20% lignin by chemical alteration and washing of lignocellulosic biomass material under elevated pressure and temperature. The method includes the steps of providing a lignocellulosic feedstock having an average constituent thickness of at most 1xe2x80x3, (most preferably up to xe2x85x9xe2x80x3 thick), introducing the feedstock into a pressure vessel having at least two reaction zones, heating the feedstock in a first reaction zone to a temperature of from about 180xc2x0 C. to about 240xc2x0 C., transferring said heated feedstock from said first reaction zone to said second reaction zone while subjecting said feedstock to an oxidizing counterflow of hot wash water of pH from about 8 pH to about 13 pH to create a residual solid containing cellulose and a filtered wash water containing dissolved materials.
Optimum operating conditions depend somewhat on the type of biomass being treated, with process times being about 1 to 10 minutes and the weight of wash water used being about 2 to 20 times the dry weight of feedstock. In addition to an oxidizer, chemicals must be introduced as necessary to maintain a pH between about 8 and 13 in various reaction zones.